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Molecular origin of contact line friction in dynamic wetting
KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics, Biophysics.ORCID iD: 0000-0001-9160-2549
KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Applied Physics, Biophysics.ORCID iD: 0000-0002-7498-7763
2018 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, no 7, article id 074201Article in journal (Refereed) Published
Abstract [en]

A hydrophilic liquid, such as water, forms hydrogen bonds with a hydrophilic substrate. The strength and locality of the hydrogen bonding interactions prohibit slip of the liquid over the substrate. The question then arises how the contact line can advance during wetting. Using large-scale molecular dynamics simulations we show that the contact line advances by single molecules moving ahead of the contact line through two distinct processes: either moving over or displacing other liquid molecules. In both processes friction occurs at the molecular scale. We measure the energy dissipation at the contact line and show that it is of the same magnitude as the dissipation in the bulk of a droplet. The friction increases significantly as the contact angle decreases, which suggests suggests thermal activation plays a role. We provide a simple model that is consistent with the observations.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC , 2018. Vol. 3, no 7, article id 074201
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-232392DOI: 10.1103/PhysRevFluids.3.074201ISI: 000437675700001Scopus ID: 2-s2.0-85051108122OAI: oai:DiVA.org:kth-232392DiVA, id: diva2:1235687
Funder
EU, European Research Council, 258980Swedish Research Council, 2014-04505
Note

QC 20180726

Available from: 2018-07-26 Created: 2018-07-26 Last updated: 2022-06-26Bibliographically approved
In thesis
1. Molecular Processes in Dynamic Wetting: Du som saknar dator/datorvana kan kontakta Cathrine Bergh, cabergh@kth.se för information
Open this publication in new window or tab >>Molecular Processes in Dynamic Wetting: Du som saknar dator/datorvana kan kontakta Cathrine Bergh, cabergh@kth.se för information
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The spreading of liquids onto and over surfaces is a fundamental process in nature. It is present in all forms and sizes: From rivers carving through landscapes, to our blood stream transporting nutrients to cells, and even single water molecules moving through channels into these cells. We now have a good understanding of how fluid movement works inside the fluid itself. However, we do not fully understand the processes close to the contact line, where the liquid is spreading onto the surface. We are forced to make assumptions about this behaviour and none of these assumptions have yet proven to be universally valid.

As everything in nature, liquid spreading is a fundamentally molecular process. This thesis summarises my work on applying this lens to the process. By studying molecules we begin at the smallest combined building blocks of nature and do not have to make any prior assumptions of the involved processes. Instead, we simply observe their behaviour. This is accomplished through the use of molecular dynamics simulation, which are an atomistic form of computer experiments. We use a realistic model of water molecules as our base liquid, since this captures realistic effects such as hydrogen bonding which are not present when using simpler models. Combined with large-scale systems which minimise the influence of finite-size effects, we have a realistic treatment of complex liquid systems.

We find that the molecular processes of wetting have an important influence on large-scale wetting. Most importantly, the hydrogen bonding nature of water to realistic substrates yields the no-slip condition often used as a boundary condition for models of wetting. Furthermore, since molecular processes are thermal in nature they create energy barriers which impede contact line advancement. We show how these barriers are created and how they can be diminished, for example in the case of electrowetting. This highlights that understanding the molecular behaviour of fluids remains an important field of study.

Abstract [sv]

Hur vätskor breder ut sig över ytor är en grundläggande process i naturen. Den dyker upp i alla former och storleksgrader: från floder som skär genom berg, till vår blodström som levererar näring till våra celler, och till och med enstaka vattenmolekyler som rör sig genom de kanaler som celler tar in näringen från. Hur vätskor beter sig i stora flöden är sedan länge känt, men vi vet ännu inte hur de beter sig nära ytor. Istället gör vi antaganden, varav inga ännu är korrekta för alla tillämpningar.

Fundamentalt sett är en vätska som breder ut sig en molekylär process. Denna avhandling sammanfattar mitt arbete med att förstå den ur denna synvinkel. Genom att studera molekyler använder vi naturens minsta sammansatta byggstenar. Vi behöver inte göra antaganden om hur de beter sig, vi behöver bara titta. Det fönster som vi tittar igenom är molekylär dynamik-simuleringar, en atomistisk typ av datorexperiment. För att fånga verkliga effekter som vätebindningar, använder vi realistiska modeller av vattenmolekyler och ytor. Vi använder tillräckligt stora system för att se hur molekylära effekter påverkar större processer.

Vi visar med dessa metoder att molekylära processer har stor påverkan på hur vätskor flödar över ytor. En stor effekt är att vätebindningarna mellan vatten och realistiska ytor förhindrar vätskan från att glida över den, vilket är ett vanligt antagande i modeller. Vi visar också hur molekyler vid gränsen där vätskor sprider på ytor ger upphov till en energibarriär som förhindrar att vätskan enkelt sprider sig framåt. Denna barriär beskrivs i detalj och vi visar vilka effekter som kan förminska den. Detta genomlyser hur molekylära processer i vätning är en viktig ingrediens för ökad förståelse av vätskespridning i system.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. p. 58
Series
TRITA-SCI-FOU ; 2020:05
Keywords
contact lines, nanodroplets, computational physics, molecular dynamics, fluid dynamics, multi-phase flows, electrowetting
National Category
Other Physics Topics
Research subject
Biological Physics
Identifiers
urn:nbn:se:kth:diva-268935 (URN)978-91-7873-480-1 (ISBN)
Public defence
2020-04-16, Livestream: https://play.kth.se/media/t/0_mbkr2jhi, Zoom link: https://kth-se.zoom.us/j/491224121, 09:00 (English)
Opponent
Supervisors
Available from: 2020-03-23 Created: 2020-03-19 Last updated: 2023-05-15Bibliographically approved

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